Magnetic disk glass substrate

ABSTRACT

A magnetic disk glass substrate including compressive stress layers at main surfaces and a tensile stress layer between the compressive stress layers formed by chemical strengthening. When the glass substrate has a thickness of less than 0.5 mm and the tensile stress layer has a thickness L and a tensile stress of Pt (kg/mm 2 ), the following relation holds: 
       0.4 (kg/mm)≦ L·Pt ≦2.0 (kg/mm)

This is a Divisional of application Ser. No. 10/594,248 filed Sep. 25,2006, claiming priority based on PCT Application No. PCT/JP2005/005362filed Mar. 24, 2005, and U.S. Provisional Application No. 60/556,021,filed Mar. 25, 2004, the contents of all of which are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a magnetic disk glass substrate and amagnetic disk that are used in a hard disk drive (HDD) being a type ofmagnetic disk device, and methods for manufacturing the magnetic diskglass substrate and the magnetic disk.

BACKGROUND ART

As the information technology is developing, dramatic innovation oninformation technology, particularly on magnetic recording technology,is desired more and more. In a magnetic disk that is to be installed ina hard disk drive (HDD) being a type of magnetic disk device used as acomputer storage, the information recording density is being increasedrapidly, unlike in other types of magnetic recording media, such asmagnetic tapes and flexible disks. Accordingly, the capacity for storinginformation in a personal computer is dramatically increasing on thestrength of the increase in information recording density of themagnetic disk.

The magnetic disk includes a magnetic layer and other layers on asubstrate, such as an aluminum-based alloy substrate or a glasssubstrate. In a hard disk drive, the magnetic disk is rapidly spun undera flying magnetic head, and the magnetic head records informationsignals as magnetized patterns on the magnetic layer, or reproduces therecorded information signals.

As the demand (for mobile use) that the hard disk drive is used inportable apparatuses (for example, notebook personal computer devices)increases, attention is directed to glass substrates having highstrength, high stiffness, and high impact resistance for the magneticdisk substrate. In addition, the glass substrate can have a smoothsurface. Accordingly the glass substrate facilitates the reduction ofthe flying height of the magnetic head that records and reproducesinformation while being floating over the magnetic disk. Thus, amagnetic disk having a high information recording density can beachieved.

However, the glass substrate is made of a brittle material. Accordingly,a variety of approaches have been proposed to strengthen the glasssubstrate. For example, Japanese Patent (JP-B) No. 2657967 (ReferenceDocument 1) has described chemical strengthening treatment in which theglass substrate is immersed in a mixed solution of KNO₃ and NaNO₃ for apredetermined time to substitute K⁺ ions for Li⁺ ions at the surfaces ofthe glass substrate and thus to form compressive stress layers at bothmain surfaces and a tensile stress layer between the compressive stresslayers. Reference Document 1 has taught that the maximum tensile stressof the tensile stress layer is preferably 4 kg/mm² or less.

Japanese Patent (JP-B) No. 3254157 (Reference Document 2) has disclosedthat when the glass substrate chemically strengthened by the same methodas in Reference Document 1 has a thickness of 0.5 to 1.0 mm, thecompressive stress layers preferably have a thickness of 80 to 100 μmand a compressive stress of 2 to 15 kg/mm² with the tensile stress layerhaving a tensile stress of 1.5 kg/mm² or less.

The information recording density of the magnetic disk has beenincreased as high as over 40 gigabits per square inch in recent years.Further, a super high recording density of over 100 gigabits per squareinch is about to be realized. The recent magnetic disk exhibiting such ahigh information recording density can store a practically sufficientamount of information even if it has a much smaller area than knownmagnetic disks.

The magnetic disk has a much higher information recording andreproduction speed (response speed) than other information recordingmedia, and can accordingly record and reproduce information anytime.

These features of the magnetic disk arouse a demand for such a smallhard disk drive as can be installed in portable apparatuses much smallerthan personal computers and requiring high response speed, such ascellular phones, digital cameras, portable information apparatuses (forexample, PDA's (personal digital assistants)), and car navigationsystems. Specifically, highly mobile apparatuses require a small harddisk drive containing a magnetic disk using a substrate having, forexample, a diameter of 50 mm or less, or of 30 mm or less, and athickness of less than 0.5 mm, or of 0.4 mm or less, and such highlymobile apparatuses include portable information apparatuses such ascellular phones, digital cameras, portable MP3 players, and PDA's, andvehicle-mounted apparatuses such as car navigation systems.

The small hard disk drive used in these portable or mobile apparatusesis always exposed to the risk of impact from falling or vibration.Accordingly, the hard disk drive for these applications, including themagnetic disk, requires that each internal member has higher impactresistance so as to enhance the reliability.

The magnetic disk using a glass substrate is useful for hard disk drivesused in the portable apparatuses. This is because the hard substratemade of glass has higher stiffness than metals, which are rather soft,and because the strength of the glass substrate can be increased to adesired level by chemical strengthening treatment or the like asdescribed above.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

Accordingly, an object of the present invention is to provide a magneticdisk glass substrate suitably used for a small hard disk drive capableof being installed in highly mobile apparatuses, such as cellularphones, digital cameras, portable MP3 players, PDA's and other portableinformation apparatuses, and car navigation systems and othervehicle-mounted apparatuses.

Another object of the invention is to provide a magnetic disk glasssubstrate capable of achieving a small hard disk drive that is notbroken even if an impact of, for example, 2000 G or more is applied, andthat allows the magnetic head to float low, for example, at a flyingheight of 10 nm or less.

Still another object of the invention is to provide a magnetic diskusing the magnetic disk glass substrate.

Means for Solving the Problems

The inventors of the present invention have found a causality betweenproblems in magnetic disks resulting from fractures caused in hard diskdrives during the drop test or the like and the manufacture process ofmagnetic disk glass substrates, particularly a chemical strengtheningstep, and have conducted intensive research to enhance the impactresistance of the glass substrate.

The inventors found a variety of difficulties in manufacture of a smallglass substrate intended for use in small hard disk drives, incomparison with the manufacture of glass substrates for generally knownso-called 2.5 inch hard disk drives or 3.5 inch hard disk drives.

Specifically, the inventors found that it is difficult to sufficientlyenhance the strength of small and thin glass substrates with a diameterof 50 mm or less and a thickness of less than 0.5 mm by knowntechniques, and that problems in magnetic disks, such as fractures,cannot be certainly prevented in some cases.

As a result of successive research, the inventors found that theabove-describe problems can be overcome by appropriately controllingchemical strengthening conditions in the manufacture of the magneticdisk glass substrate.

While the impact resistance of the glass substrate may be enhanceddepending on the conditions of the chemical strengthening treatment, thechemical strengthening treatment increases the waviness (“Wa” inabbreviated form) at the surfaces of the substrate. Consequently, theglide height of the magnetic disk using the glass substrate may beundesirably increased and the reduction of the flying height may beimpaired.

In order to simultaneously overcome the two problems with the impactresistance of the glass substrate and the waviness (Wa) at the surfaceof the substrate, the inventors further advanced the research, and foundthat the two problems can be overcome at one time by appropriatelysetting chemical strengthening conditions to control the chemicalstrengthening treatment.

The present invention includes the following aspects.

(First Aspect)

A magnetic disk glass substrate according to the present invention has adisk thickness of less than 0.5 mm so as to be used in 1 inch hard diskdrives or smaller hard disk drives using a smaller magnetic disk thanthat of the 1 inch hard disk drive. The glass substrate has apredetermined disk thickness and mirror-finished main surfaces with nocracks, by lapping the main surfaces. The glass substrate is subjectedto chemical strengthening treatment to form compressive stress layerswith thicknesses d1 and d2 at the main surfaces and a tensile stresslayer with a thickness L between the compressive stress layers. Theproduct L·Pt of the thickness L of the tensile stress layer and themaximum tensile stress Pt of the tensile stress layer is set at apredetermined value, so that the magnetic disk glass substrate has apredetermined impact resistance and a predetermined waviness (Wa) at themain surfaces. The thickness L is measured by observing longitudinalsection of the magnetic disk glass substrate with a Babinet compensator.

For example, the predetermined impact resistance may be 3000 G or more,and the predetermined waviness (Wa) may be 1.0 nm or less.

(Second Aspect)

In the magnetic disk glass substrate of the first aspect of the presentinvention, the product L·Pt of the thickness L of the tensile stresslayer and the tensile stress Pt of the tensile stress layer is in therange of 0.4 to 2.0 kg/mm.

Herein, the magnetic disk glass substrate satisfies the followingrelationship:

0.4 (kg/mm)≦L·Pt≦2.0 (kg/mm)

Although the thickness L of the tensile stress layer may be derived fromthe equation L={T−(d1+d2)} using the disk thickness T after the chemicalstrengthening treatment, the tensile stress layer thickness L ispreferably directly measured.

(Third Aspect)

In the magnetic disk glass substrate of the first aspect of the presentinvention, the tensile stress layer thickness L is 0.4 mm or less, andthe maximum tensile stress Pt of the tensile stress layer is 10 kg/mm²or less.

Herein, the magnetic disk glass substrate satisfies the followingrelationship:

L≦0.4 (mm)

Pt≦10 (kg/mm²)

(Fourth Aspect)

In the magnetic disk glass substrate of the first aspect of the presentinvention, one of the compressive stress layers at the main surfaces hasa thickness d1, the other compressive stress layer has a thickness d2,and the total thickness D of the thicknesses d1 and d2 is 40% or more ofthe disk thickness T.

Herein, the magnetic disk glass substrate satisfies the followingrelationship:

(D/T)≧0.4

(Fifth Aspect)

In the magnetic disk glass substrate of the fourth aspect of the presentinvention, the maximum tensile stress Pt of the tensile stress layer is10 kg/mm² or less.

Herein, the magnetic disk glass substrate satisfies the followingrelationship:

Pt≦10 (kg/mm²)

(Sixth Aspect)

In the magnetic disk glass substrate of the first aspect of the presentinvention, the compressive stress layer has a maximum compressive stressPc of 4 kg/mm² or more.

Herein, the magnetic disk glass substrate satisfies the followingrelationship:

Pc≧4 (kg/mm²)

(Seventh Aspect)

The magnetic disk glass substrate of the first aspect of the presentinvention is used for a magnetic disk installed in a hard disk drivethat starts and stop operation by a load/unload system.

(Eighth Aspect)

A magnetic disk of the present invention includes the magnetic diskglass substrate according to the first aspect, and at least a magneticlayer is formed on the magnetic disk glass substrate.

(Ninth Aspect)

A method for manufacturing the magnetic disk glass substrate accordingto the present invention includes the chemical strengthening step ofbringing a glass substrate into contact with a melted mixture of atleast three alkali metal nitrates to perform low-temperature ionexchange, thereby forming the compressive stress layers at both mainsurfaces of the glass substrate and the tensile stress layer between thecompressive stress layers.

(Tenth Aspect)

The method for manufacturing the magnetic disk glass substrate of theninth aspect further includes the polishing step of relatively moving anabrasive cloth and the glass substrate while colloidal silica abrasivegrain or diamond abrasive grain is fed, thereby removing cracks in themain surfaces of the glass substrate to form mirror-finished surfaces.

(Eleventh Aspect)

In the method for manufacturing the magnetic disk glass substrate of thetenth aspect, the mirror-finished surfaces formed in the polishing stephas an arithmetic mean roughness (Ra) of 0.4 nm or less.

(Twelfth Aspect)

A method for manufacturing a magnetic disk includes the step of formingat least a magnetic layer on a main surface of the magnetic disk glasssubstrate manufactured by the method according to the ninth aspect.

Effect of the Invention

In a magnetic disk glass substrate having compressive stress layersrespectively having thicknesses d1 and d2 formed at both main surfacesby chemical strengthening treatment and a tensile stress layer having athickness L and a maximum tensile stress Pt, by setting the product L·Ptof the tensile stress layer thickness and the tensile stress at apredetermined value, the magnetic disk glass substrate can have apredetermined impact resistance, and a predetermined waviness (Wa) atthe main surfaces.

The predetermined impact resistance and the predetermined waviness (Wa)at the main surfaces are such values (for example, an impact resistanceof 3000 G or more; Wa of 1.0 nm or less) as do not cause problems whenthe magnetic disk glass substrate with a disk thickness of less than 0.5mm is used in a 1 inch hard disk drive or a hard disk drive containing asmaller magnetic disk than that of the 1 inch hard disk drive.

When the product L·Pt of the tensile stress layer thickness and themaximum tensile stress is in the range of 0.4 to 2.0 kg/mm under theconditions where the tensile stress layer has a thickness L and amaximum tensile stress Pt, the magnetic disk glass substrate can have afavorable impact resistance, and a favorable waviness (Wa) at thesurfaces.

By setting the thickness L of the tensile stress layer at 0.4 mm orless, or by setting the total thickness D of the thickness d1 of onecompressive stress layer formed at one of the main surfaces and thethickness d2 of the other compressive stress layer at 40% or morerelative to the disk thickness T, the magnetic disk glass substrate canhave a favorable impact resistance.

By setting the maximum tensile stress Pt of the tensile stress layer at10 kg/mm² or less, the magnetic disk glass substrate can have afavorable impact resistance and durability, and a favorable waviness(Wa) at the surfaces.

By setting the highest compressive stress Pc of the compressive stresslayer at 4 kg/mm² or more, the magnetic disk glass substrate can have afavorable impact resistance.

Since the magnetic disk of the present invention includes the magneticdisk glass substrate and at least the magnetic layer on the glasssubstrate, it has a favorable impact resistance and durability. Themagnetic disk can be installed in a hard disk drive that starts andstops operation by a load/unload system.

Accordingly, the present invention can be suitably applied to small harddisk drives that can be installed in highly mobile apparatuses, such ascellular phones, digital cameras, portable MP3 players, PDA's and otherportable information apparatuses, and car navigation systems andvehicle-mounted apparatuses. A magnetic disk glass that is not brokeneven if an impact of 2000 G or more is applied can be used in the harddisk drives, and a magnetic disk using such a magnetic disk glasssubstrate can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the profile of stress layers at asection of a magnetic disk glass substrate according to the presentinvention.

FIG. 2 is a plot of the relationship between the maximum tensile stressPt of the tensile stress layer and the thickness L of the tensile stresslayer of magnetic disk glass substrates subjected to chemicalstrengthening treatment under various conditions.

FIG. 3 is a plot of the relationship between the waviness (Wa) at thesurfaces of glass substrates subjected to chemical strengtheningtreatment under various conditions and the glide height of magneticdisks using the glass substrates.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be described indetail with reference to the drawings.

In order to produce the magnetic disk glass substrate of the presentinvention, the main surfaces of a glass plate is lapped (ground) toprepare a glass base. The glass base is cut into a glass disk, and thenthe main surfaces of the glass disk are polished.

The glass plate to be lapped in the present invention may have a varietyof shapes, and may have a rectangular shape or a disk shape. Disk-shapedglass plates can be lapped with a lapping machine conventionally used inthe manufacture of magnetic disk glass substrates, and can be reliablyprocessed at low cost.

The glass plate must be larger than the desired magnetic disk glasssubstrate. For example, a magnetic disk glass substrate used in amagnetic disk installed in a 1 inch hard disk drive or a smaller harddisk drive has a diameter of about 10 to 30 mm. Accordingly, thedisk-shaped glass plate has a diameter of 30 mm or more, and preferably48 mm or more. In particular, a disk-shaped glass plate with a diameterof 65 mm or more can be formed into a plurality of magnetic disk glasssubstrates intended for use in 1 inch hard disk drives, and is thuspreferable in view of mass production. The upper limit in size of theglass plate is not particularly limited, but the disk-shaped glass platepreferably has a diameter of 100 mm or less.

The glass plate can be made of, for example, molten glass by a knownprocess, such as pressing, floating, or fusion. Among these processes,pressing can provide the glass plate at low cost.

Any glass can be used for the glass plate as long as it can bechemically strengthened. Preferably, aluminosilicate glass is used.Lithium-containing aluminosilicate glass is particularly preferable.Aluminosilicate glass facilitates precise formation of compressivestress layers having an appropriate compressive stress and a tensilestress layer having an appropriate tensile stress by ion-exchangechemical strengthening treatment, particularly low-temperatureion-exchange chemical strengthening treatment.

Preferably, the aluminosilicate glass has a composition mainlycontaining 58% to 75% by weight of SiO₂, 5% to 23% by weight of Al₂O₃,3% to 10% by weight of Li₂O, and 4% to 13% by weight of Na₂O.

More preferably, the aluminosilicate glass has a composition mainlycontaining 62% to 75% by weight of SiO₂, 5% to 15% by weight of Al₂O₃,4% to 10% by weight of Li₂O, 4% to 12% by weight of Na₂O, and 5.5% to15% by weight of ZrO₂, with a Na₂O/ZrO₂ weight ratio of 0.5 to 2.0 and aAl₂O₃/ZrO₂ weight ratio of 0.4 to 2.5.

In order to remove protrusions resulting from undissolved ZrO₂ at thesurface of the glass disk, a glass suitable for chemical strengtheningtreatment is preferably used which contains 57% to 74% of SiO₂, 0 to2.8% of ZrO₂, 3% to 15% of Al₂O₃, 7% to 16% of Li₂O, and 4% to 14% ofNa₂O, on a molar basis.

Such aluminosilicate glass can be chemically strengthened to increasethe flexural strength and Knoop hardness.

The lapping is intended to increase the profile precision (for example,flatness of the main surfaces) and dimensional precision (for example,precision in thickness) of the main surfaces of the workpiece or theglass plate. The lapping is performed by relatively moving the glassplate and a grinding stone or surface plate to grind the main surface ofthe glass plate, with the grinding stone or surface plate pressedagainst the surface of the glass plate. The lapping can be performedwith a double side lapping machine using a planetary gear system.

For the lapping, the main surfaces of the glass plate are preferably fedwith a grinding fluid to wash sludge (swarf) out of the ground surfaces,and besides to cool the ground surfaces. A slurry prepared by addingfree abrasive grain to the grinding fluid may be fed to the mainsurfaces of the workpiece for the lapping.

The grinding stone used for the lapping may be a diamond grinding stone.Further, hard abrasive grain, such as that of alumina, zirconia, orsilicon carbide, is preferably used as free abrasive grain.

The lapping improves the profile precision of the glass plate toplanarize the main surfaces and reduces the thickness of the glass plateto prepare a glass base with a predetermined thickness.

In the present invention, the main surfaces of the glass base areplanarized and the thickness is reduced, by the lapping. Thus, the glassdisk can be cut out from the glass base. More specifically, the presentinvention can prevent defects, such as chipping, cracking, andfracturing, that may occur when the magnetic disk is cut out from theglass base.

For example, the flatness for a glass base with an area of 7088 mm²(area of a circle with a diameter of 95 mm) is preferably 30 μm or less,and more preferably 10 μm or less. The flatness mentioned herein refersto the largest value of the waves with wavelengths of, for example, 200nm to 95 mm in the entire circle of the glass base with a diameter of 95mm, measured with an apparatus OPTIFLAT (product name) manufactured byPHASE SHIFT TECHNOLOGY or its equivalent. Preferably, the thickness ofthe glass base is 2 mm or less, and more preferably 0.8 mm or less. Aglass base with a thickness of less than 0.2 mm may not endure the loadapplied when the glass disk is cut out from the glass base. Therefore,the glass base preferably has a thickness of 0.2 mm or more. On theother hand, a glass base with a thickness of more than 2 mm may not beprecisely cut because of its large thickness, and may cause a defect,such as chip, crack, or fracture, when the glass disk is cut out.

The glass base must be larger than the desired magnetic disk glasssubstrate. For example, for a magnetic disk used in a 1 inch or asmaller hard disk drive, a magnetic disk glass substrate with a diameterof about 10 to 30 mm is used. Accordingly, the glass base has a diameterof 30 mm or more, and preferably 48 mm or more. In particular, a glassbase with a diameter of 65 mm or more can provide a plurality ofmagnetic disk glass substrates intended for use in 1 inch hard diskdrives, and is thus preferable in view of mass production. The upperlimit in size of the glass base is not particularly limited, but thedisk-shaped glass base preferably has a diameter of 100 mm or less.

For cutting the glass base, a cutting blade or grinding stone made ofharder material than glass can be used, such as a diamond cutter or adiamond drill. Alternatively, a laser cutter may be used to cut theglass base. However, small glass disks with diameters of 30 mm or lessare often difficult to cut out with a laser cutter. In this case, suchglass disks can be easily cut out with a cutting blade or grindingstone.

The glass disk used in the invention prepared from the glass basepreferably has a diameter of 30 mm or less. In the present invention,the glass disk that has been subjected to the lapping is at leastpolished to mirror-finish the main surfaces of the glass disk.

The polishing removes cracks in the main surfaces of the glass disk, andthe surface roughness of the main surfaces measured by atomic forcemicroscopy (AFM) is reduced to 5 nm or less in terms of R_(max), or 0.4nm or less in terms of arithmetic mean roughness (Ra). The values of thesurface roughness are calculated from the surface geometry measured byAFM in accordance with Japan Industrial Standard (WS) B0601. The glassdisk having the mirror-finished main surfaces can prevent problemsresulting from a so-called head crash or thermal asperities even if themagnetic disk using the glass disk includes a magnetic head having aflying height of, for example, 10 nm. The mirror-polished main surfacesof the glass disk facilitate uniform chemical strengthening treatmenteven in microfabricated regions, and prevent microcracks and, thus,delayed fracture.

For the polishing, for example, a surface plate with an abrasive cloth(for example, polishing pad) is pressed against the main surface of theglass disk, and the glass disk and the surface plate are relativelymoved while polishing liquid is being fed to the surface of the glassdisk. The polishing liquid preferably contains abrasive grain. Forexample, colloidal silica grain can be used as the abrasive grain.Preferably, the abrasive grain has an average grain size of 10 to 200nm.

Alternatively, a technique using an abrasive tape may be applied inwhich, for example, a tape-shaped abrasive cloth (for example, abrasivetape) is pressed against the main surface of the glass disk, and theglass disk and the abrasive cloth are relatively moved while polishingliquid is being fed to the main surface of the glass disk. The polishingliquid preferably contains abrasive grain. For example, diamond abrasivegrain can be used as the abrasive grain. Preferably, the abrasive grainhas an average grain size of 10 to 200 nm.

The abrasive surface of the polishing pad or abrasive tape used in thepresent invention is preferably formed of a resin, such as polyurethaneor polyester. Preferably, the polishing pad has an abrasive surfaceformed of a resin foam (for example, polyurethane foam), and theabrasive tape has an abrasive surface formed of resin fiber (forexample, polyester fiber).

In the present invention, preferably, the glass disk is subjected tolapping before polishing. This lapping is performed in the same manneras the lapping of the glass plate. By lapping the glass disk beforepolishing, mirror-finished main surfaces can be formed in a shortertime.

In the present embodiment, preferably, the periphery of the glass diskis mirror-polished. The periphery of the glass disk is coarse and in acut state, and the periphery is polished into a mirror-finished surface.Thus, particulate matter is prevented, and failures resulting fromthermal asperities can be appropriately prevented in the magnetic diskusing the magnetic disk glass substrate. In addition, themirror-polished surface can prevent delayed fracture resulting frommicrocracks. Preferably, the mirror-polished periphery has an arithmeticmean roughness (Ra) of 100 nm or less when measured by AFM.

In the present invention, chemical strengthening treatment is performedbefore and after the step of polishing the glass disk. The chemicalstrengthening treatment produces a high compressive stress at thesurfaces of the magnetic disk glass substrate to increase the impactresistance. In particular, a glass disk made of aluminosilicate glass ischemically strengthened favorably.

Any known chemical strengthening technique can be applied to thechemical strengthening treatment without particular limitation. Theglass disk is chemically strengthened by, for example, bringing theglass disk into contact with a heated chemical strengthening molten saltto perform ion exchange substituting the ions of the chemicalstrengthening salt for the ions at the surface of the glass disk.

Ion exchange may be performed by a known method, such as low-temperatureion exchange, high-temperature ion exchange, surface crystallization, orglass surface dealkalization. Preferably, a low-temperature ion exchangemethod is applied. The low-temperature ion exchange method is performedat a temperature of the annealing point or less of the glass.

In the low-temperature ion exchange, alkali metal ions in the glass arereplaced with other alkali metal ions having a larger ion radius thanthe alkali metal ions in the glass at a temperature of the annealingpoint or less. Consequently, the volume of the ion-exchanging portion isincreased to produce a compressive stress at the surfaces of the glass.Thus, the surfaces of the glass are strengthened.

In the chemical strengthening treatment, the molten salt is heated to atemperature of 280 to 660° C., particularly 300 to 400° C. in order toconduct appropriate ion exchange.

The time for which the glass disk is in contact with the molten salt ispreferably several hours to tens of hours.

Preferably, the glass disk is preheated to a temperature of 100 to 300°C. before coming into contact with the molten salt. After the chemicalstrengthening treatment, the glass disk is cooled and cleaned, and thus,a final product (magnetic disk glass substrate) is completed.

The chemical strengthening bath for the chemical strengthening treatmentcan be made of any material without particular limitation as long as ithas high corrosion resistance and does not produce dust. This is becausethe chemical strengthening salt or chemical strengthening molten salthas oxidizing properties, and because this treatment is performed at ahigh temperature. Use of highly corrosion-resistant material preventsdamage and dust, and thus prevents failures resulting from thermalasperities and head crash. Accordingly, the chemical strengthening bathis preferably made of quartz. Stainless steel may be used, includingcorrosion-resistant martensitic stainless steel and austenitic stainlesssteel. Quartz is superior in corrosion resistance, but is expensive. Anappropriate material may be selected in view of profitability.

The chemical strengthening salt used in the present invention preferablycontains a nitrate of an alkali metal element, such as potassiumnitrate, sodium nitrate, or lithium nitrate. If the nitrate containslithium, the lithium content is preferably 10 to 3000 ppm (for a mixtureof three types of nitrates: potassium nitrate, sodium nitrate, andlithium nitrate, the mixture contains 0.001% to 0.3% by volume oflithium nitrate). If the lithium ion content in the chemicalstrengthening molten salt is excessively high, ion exchange isinhibited. Consequently, it may become difficult to obtain desiredtensile stress and compressive stress. By chemically strengtheningglass, particularly lithium-containing aluminosilicate glass, with thechemical strengthening salt, the resulting magnetic disk glass substratecan have a desired stiffness and impact resistance, and desired waviness(Wa) at the surface of the substrate.

The thus produced magnetic disk glass substrate of the present inventionis suitable for a thin magnetic disk with a disk thickness of less than0.5 mm, particularly of 0.1 to 0.4 mm. Further, the magnetic disk glasssubstrate is suitable for a small magnetic disk with a diameter (outerdiameter) of 30 mm or less. Such a thin or small magnetic click isinstalled in a 1 inch hard disk drive or a hard disk drive smaller thanthe 1 inch hard disk drive. Thus, the magnetic disk glass substrate issuitable for 1 inch hard disk drives and smaller hard disk drives thanthe 1 inch hard disk drives.

For the magnetic disk installed in the 1 inch hard disk drive, themagnetic disk glass substrate has a diameter of about 27.4 mm and a diskthickness of 0.381 mm. For the magnetic disk installed in the 0.85 inchhard disk drive, the magnetic disk glass substrate has a diameter ofabout 21.6 mm and a disk thickness of 0.381 mm.

A magnetic disk according to the present invention has a magnetic layeron the magnetic disk glass substrate. For example, the magnetic layermay be formed of cobalt (Co)-based ferromagnetic material. Inparticular, the magnetic layer is preferably made of cobalt-platinum(Co—Pt) or cobalt-chromium (Co—Cr) ferromagnetic material that canproduce a high coercive force. The magnetic layer can be formed by DCmagnetron sputtering.

An underlayer or the like may be formed between the glass substrate andthe magnetic layer, if necessary. The underlayer can be formed of anAl—Ru alloy or a Cr-based alloy.

The magnetic layer may be covered with a protective layer for protectingthe magnetic disk against the impact from the magnetic head. Theprotective layer is preferably formed of a hard hydrogenated carbonfilm.

In addition, a PFPE (perfluoro polyether) lubricating layer may beformed over the protective layer to alleviate the interference betweenthe magnetic head and the magnetic disk. The lubricating layer can beformed by, for example, dipping.

EXAMPLES

The present invention will be further described in detail with referenceto Examples. However, the invention is not limited to the form of theExamples.

Example 1

A method for manufacturing the magnetic disk glass substrate in thepresent Example includes the following steps (1) to (7):

(1) rough lapping step (rough grinding step);

(2) shaping step (peripheral lapping step);

(3) precision lapping step (precision grinding step);

(4) peripheral mirror-polishing step;

(5) first polishing step;

(6) second polishing step; and

(7) chemical strengthening step.

First, a disk-shaped amorphous aluminosilicate glass base was prepared.This aluminosilicate glass contained lithium. Specifically, thealuminosilicate glass had a composition of 63.6% by weight of SiO₂,14.2% by weight of Al₂O₃, 10.4% by weight of Na₂O, 5.4% by weight ofLi₂O, 6.0% by weight of ZrO₂, and 0.4% by weight of Sb₂O₃.

(1) Rough Lapping Step

A 0.6 mm thick glass sheet made from molten aluminosilicate glass wasused as the glass base. The glass sheet was formed into a disk-shapedglass disk with a diameter of 28.7 mm and a thickness of 0.6 mm using agrinding stone.

The glass sheet is generally formed by a down draw process or a floatprocess. The disk-shaped glass base may be prepared by direct press. Anyaluminosilicate glass can be used as the material of the glass sheet, aslong as containing 58% to 75% by weight of SiO₂, 5% to 23% by weight ofAl₂O₃, 4% to 13% by weight of Na₂O, and 3% to 10% by weight of Li₂O.

Then, the glass disk was subjected to the lapping step in order toincrease the dimensional precision and profile precision. The lappingstep was performed using a double side lapping machine with abrasivegrain of #400 in grain size.

Specifically, both surfaces of the glass disk housed in a carrier werelapped to a flatness of 0 to 2 μm and a surface roughness (R_(max)) ofabout 6 μm with alumina abrasive grain of #400 in grain size by rotatinga sun gear and an internal gear at a load of about 100 kg. The flatnessrefers to the largest value of the waves with wavelengths of 200 nm to28.7 mm and is measured with an apparatus OPTIFLAT (product name)manufactured by PHASE SHIFT TECHNOLOGY. The surface roughness (R_(max))was measured with a surface roughness meter based on the tracer method.

(2) Shaping Step

Then, a hole was formed in the center of the glass disk using acylindrical grinding stone, and the edge of the periphery of the glassdisk was ground. Subsequently, the edges of the periphery and the innerwall of the glass disk were chamfered in a predetermined manner. Thesurface roughness of the edges at this point was about 4 μm in terms ofR_(max) measured by the tracer method.

(3) Precision Lapping Step

Then, the main surfaces of the glass disk were lapped to a diskthickness of 0.427 mm, a flatness of 0 to 2 μm, surface roughnessR_(max) of about 2 μm, and a surface roughness Ra of about 0.2 μm withabrasive grain of #1000 in grain size. The flatness refers to thelargest value of the waves with wavelengths of 200 nm to 28.7 mm, and ismeasured with OPTIFLAT (product name) manufactured by PHASE SHIFTTECHNOLOGY. The surface roughness (R_(max), Ra) was measured with asurface roughness meter based on the tracer method.

The precision lapping step can reduce fine roughness formed at the mainsurfaces in the foregoing rough lapping step and shaping step.

After the precision lapping step, the glass disk was subjected toultrasonic cleaning in cleaning baths of a neutral detergent and water,in that order, to which ultrasonic waves were applied.

(4) Peripheral Mirror-Polishing Step

The edges of the peripheries (inner periphery and outer periphery) ofthe glass disk were polished to a surface roughness Ra of about 40 nmwith a brush while the glass disk was rotated. The surface roughness(Ra) was measured by AFM.

After the peripheral mirror-polishing, the main surfaces of the glassdisk were rinsed with water.

In the peripheral mirror-polishing step, glass disks are stacked andtheir edges are polished. In order to prevent surface flaws at the mainsurfaces of the glass disks, the peripheral mirror-polishing step ispreferably performed before the below-described first polishing step, orbefore and after the second polishing step.

The edges of the glass disk were mirror-finished by the peripheralmirror-polishing step so as to prevent dust such as particulate matterfrom being generated. After the peripheral mirror-polishing step, thediameter of the glass disk was measured and the result was 27.4 mm.

(5) First Polishing Step

Then, the first polishing step was performed with a double sidepolishing machine to remove residual flaws and strain.

In the double side polishing machine, the glass disk held by a carrierwas allowed to adhere between an upper and a lower surface plate towhich polishing pads are bonded. The carrier was engaged in a sun gearand an internal gear with the glass disk pressed between the upper andlower surface plates. Then, the sun gear was rotated so that the glassdisk rotates on its axis and around the internal gear between thesurface plates, while a polishing liquid was fed between the abrasivesurfaces of the polishing pads and the main surfaces of the glass disk.Thus, the main surfaces were polished at one time.

The same double side polishing machine was used in the followingExamples. Specifically, the first polishing step was performed usingpolyurethane foam as the polishing pad, and a polishing liquidcontaining cerium oxide and RO water. After the first polishing step,the glass disk was cleaned using ultrasonic technique in cleaning bathsof a neutral detergent, pure water (1), pure water (2), and IPA(isopropyl alcohol) in that order, followed by drying in an IPA vaporbath.

(6) Second Polishing Step

Then, the second polishing step was performed. In this step, the mainsurfaces were mirror-polished with a soft polishing pad (made ofpolyurethane foam) and the same double side polishing machine as used inthe first polishing step.

The second polishing step is carried in order to remove cracks certainlyand to obtain mirror-finish the main surfaces reduced the surfaceroughness Ra of the main surfaces to, for example, about 0.4 to 0.1 nmwhile maintaining the flat main surfaces formed by the first polishingstep. In this case, the surface roughness Ra is measured by AFM.

More specifically, the second polishing step was performed at a load of100 g/cm² for 5 minutes, using a polishing liquid containing colloidalsilica grains (average grain size: 80 nm) and RO water.

After the second polishing step, the glass disk was cleaned usingultrasonic technique in cleaning baths of a neutral detergent, purewater (1), pure water (2), and IPA (isopropyl alcohol) in that order,followed by drying in an IPA vapor bath.

(7) Chemical Strengthening Step

After the cleaning, the glass disk was subjected to chemicalstrengthening treatment. The chemical strengthening treatment used achemical strengthening molten salt prepared by mixing 59.9995% by volumeof potassium nitrate, 39.9995% by volume of sodium nitrate, and 0.001%by volume of lithium nitrate. The lithium content measured with an ICPemission analyzer was 10 ppm.

The glass disk after cleaning and drying was subjected to chemicalstrengthening treatment by immersing the disk in the chemicalstrengthening solution heated to 340° C. for 2 hours. In order tochemically strengthen the entire surface of the resulting magnetic diskglass substrate during immersion, a plurality of magnetic disk glasssubstrates were housed in a holder with their peripheries held.

After the chemical strengthening treatment, the magnetic disk glasssubstrate was rapidly cooled in a water bath of 20° C. for about 10minutes.

After cooling, the magnetic disk glass substrate was cleaned byimmersing in concentrated sulfuric acid heated to about 40° C.Subsequently, the glass substrate was further cleaned using ultrasonictechnique in cleaning baths of pure water (1), pure water (2), and IPA(isopropyl alcohol) in that order, followed by drying in an IPA vaporbath.

Then, the main surfaces of the magnetic disk glass substrate weresubjected to visual inspection and subsequently thorough preciseinspection by optical reflection, scattering, and transmission. As aresult, protrusions resulting from trapped foreign matter or flaws werenot found at the main surfaces of the magnetic disk glass substrate.

In addition, the surface roughness of the magnetic disk glass substratewas measured by AFM. As a result, it was confirmed that extremely smoothmirror-finished surfaces were formed with an R_(max) of 2.5 nm and an Raof 0.30 nm. The values representing the surface roughness werecalculated in accordance with Japan Industrial Standard (WS) B0601 fromthe surface geometry measured by AFM.

Further, the magnetic disk glass substrate had an inner diameter of 7mm, an outer diameter of 27.4 mm, and a thickness of 0.381 mm. Thismeans that the magnetic disk glass substrate has suitable dimensions foruse in 1 inch magnetic disk.

In addition, the inner periphery defining the hole in the magnetic diskglass substrate had surface roughnesses of 40 nm in terms of Ra measuredby AFM at the edge, and of 50 nm in terms of Ra at the inner wall. Theexternal periphery had surface roughness of 40 nm in terms of Ra at thechamfered portion, and of 70 nm in terms of Ra at the external wall.Thus, it was confirmed that the internal periphery was mirror-finishedas well as the external periphery.

The main surfaces of the resulting magnetic disk glass substrate wereanalyzed with an electron microscope. As a result, it was confirmed thatthe main surfaces had been mirror-finished with no cracks. Bymirror-polishing the main surfaces with colloidal silica abrasive grain(average grain size: 80 nm), smooth mirror-finished surfaces with an Raof 0.30 nm were formed.

The mirror-finished main surfaces with an Ra of about 0.1 to 0.4 nm andwith no cracks certainly prevent delayed fracture in the chemicallystrengthened glass.

Furthermore, the resulting magnetic disk glass substrate did not haveforeign matter or particulate matter that may cause thermal asperitieson the surfaces, nor have foreign matter or cracks on the inner wall ofthe hole.

(Measurement of Stress Layers)

The magnetic disk glass substrate was cut into a rectangular piece witha width of about 3 mm so as to expose the cross-sections perpendicularto the main surfaces. Then, both cross-sections (sections of thesubstrate) of the rectangular piece were ground and polished with anabrasive and a polishing pad until the distance between thecross-sections was reduced to about 0.5 mm.

FIG. 1 is a sectional view of the profile of the stress layers of themagnetic disk glass substrate.

The sectional profile of the stress layers as shown in FIG. 1 can beobtained by measuring the exposed section of the magnetic disk glasssubstrate with a Babinet compensator.

The Babinet compensator includes two opposing quartz wedges with thesame angle. One of the wedges is shifted in the direction of its lengthby a screw of a micrometer. The two wedges are perpendicular to theoptical axis, and the movable prism has an axis extending in theshifting direction. This instrument is widely used for inspections forthe phase difference (retardation) and the degree of double refractionof crystals, checks on glass with internal stress, and the like.

The profile of the stress layers includes the following.

T: thickness (total) of magnetic disk glass substrate (mm)

d1, d2: thickness (depth) of compressive stress layers (stress layerdepth) (mm)

D (=d1+d2): total thickness of compressive stress layers (mm)

L (=T−(d1+d2)): thickness of tensile stress layer (mm)

Pc: compressive stress (compression stress) (kg/mm²)

Pt: tensile stress (kg/mm²)

The magnetic disk glass substrate prepared in the present Example wasPt=3.62 (kg/mm²), Pc=10.10 (kg/mm²), D/2=0.089 (mm), D/T=0.47, L=0.203(mm), and L·Pt=0.735 (kg/mm).

(Impact Resistance Test)

The magnetic disk glass substrate was subjected to Dana's impact testwith AVEX-SM-110-MP manufactured by Arbrown. Specifically, impacts ofpulsed sine half waves from 1000 G to 5000 G were applied to themagnetic disk glass substrate combined with a dedicated impact test jigin the direction perpendicular to the main surfaces, and the magneticdisk glass substrate was checked for fractures.

A small hard disk drive (HDD) containing a magnetic disk using asubstrate with a diameter of 50 mm or less, or 30 mm or less, and athickness of less than 0.5 mm, or 0.4 mm or less, requires productspecifications ensuring that the hard disk drive (HDD) can endure animpact of 2000 G when it is subjected to drop test. However, theinventors found that in drop test performed on hard disk drives (HDD's)including magnetic disk glass substrates designed so as to be endurableto 2000 G in a solely subjected impact test, several percent of thesubstrates were fractured. On the other hand, substrates designed so asto be endurable to 3000 G were not fractured at all. As a consequence,in order to ensure that the completed hard disk drive (HDD) endures animpact of 2000 G, the substrate must be solely endurable to 3000 G.Accordingly, magnetic disk glass substrates solely endurable to 3000 Gin impact test were determined to be acceptable.

The magnetic disk glass substrate prepared in the present Example wasendurable to an impact of 3000 G in impact test.

(Measurement of Waviness (Wa))

The waviness (Wa) at the surface of magnetic disk glass substrate wasmeasured. The waviness (Wa) of the substrate is a property of thesurface geometry of the substrate, and is applied to waves withwavelengths on the order of millimeters and amplitudes on the order ofnanometers. Waves with smaller wavelengths are represented by“roughness” while larger waves are represented by “flatness”. The“roughness”, “waviness”, and “flatness” are each a property representingthe surface geometry of the substrate, and are difficult to discriminateclearly from each other. The surface of the glass substrate in practicehas unevenness with wavelengths and amplitudes on the order of angstroms(hereinafter referred to as “minimal unevenness”) at random. The“roughness” expresses the state of the “minimal unevenness on the orderof micrometers. Although it seems that the roughness includes minimalunevenness at random, the state where the unevenness is present has acertain periodicity in a relatively long span. The periodicity of the“minimal unevenness” is the “waviness”. Accordingly, it can be said thatthe “waviness” expresses the feature of the state where the unevennessis present.

In the present Example, the waviness (Wa) was measured with an opticalmeasuring unit OPTIFLAT (product name) manufactured by PHASE SHIFTTECHNOLOGY. For a magnetic disk glass substrate with an outer diameterof 27.4 mm, inner diameter of 7 mm, and a thickness of 0.381 mm, thewaviness (Wa) refers to the average of the waves with wavelengths of 200nm to 5 mm at the entire surface of the substrate, that is, in an areasurrounded by concentric circles with radius (r) of 3.5 mm and 13.7 mmfrom the center of the disk.

The waviness of the magnetic disk glass substrate prepared in thepresent Example was measured and the result was Wa=0.54 nm.

(Manufacture of Magnetic Disk)

The magnetic disk was manufactured through the following process.

An Al—Ru seed layer, a Cr—W underlayer, a Co—Cr—Pt—Ta magnetic layer,and a hydrogenated carbon protective layer were formed in that order oneach main surface of the magnetic disk glass substrate prepared throughthe foregoing steps by the use of a statically opposed DC magnetronsputtering apparatus. The seed layer reduces the grain size of themagnetic grains of the magnetic layer, and the underlayer orients theeasy magnetization axis of the magnetic layer in the in-plane direction.

The magnetic disk at least includes the magnetic disk glass substratebeing a nonmagnetic substrate, the magnetic layer formed on the magneticdisk glass substrate, a protective layer formed on the magnetic layer,and a lubricating layer formed on the protective layer.

The nonmagnetic metal layers (nonmagnetic underlayers) comprising theseed layer and the underlayer are formed between the magnetic disk glasssubstrate and the magnetic layer. The layers of the magnetic disk otherthan the magnetic layer are made of nonmagnetic materials. In thepresent Example, the magnetic layer was in contact with the protectivelayer, and the protective layer was in contact with the lubricatinglayer.

Specifically, at first, the Al—Ru (aluminum-ruthenium) seed layer wasdeposited to a thickness of 30 nm on the magnetic disk glass substrateby sputtering with an Al—Ru alloy target (Al: 50 at %, Ru: 50 at %).Then, the Cr—W (chromium-tungsten) underlayer was deposited to athickness of 20 nm on the seed layer 5 by sputtering with a Cr—W alloytarget (Cr: 80 at %, W: 20 at %). Then, the Co—Cr—Pt—Ta(cobalt-chromium-platinum-tantalum) magnetic layer was deposited to athickness of 15 nm on the underlayer by sputtering with a Co—Cr—Pt—Taalloy target (Cr: 20 at %, pt: 12 at %, Ta: 5 at %, balance being Co).

Then, the magnetic layer was coated with the hydrogenated carbonprotective layer, and further the PFPE (perfluoro polyether) lubricatinglayer was formed by dipping. The protective layer protects the magneticlayer against the impact from the magnetic head. Thus, the magnetic diskwas obtained.

The resulting magnetic disk was subjected to a glide height test using aAE sensor. The glide height of the magnetic disk was 4.3 nm. Further,the magnetic disk was subjected to a glide test using a glide head at aflying height of 10 nm. No foreign matter coming into contact with themagnetic disk was found, and a stable floating state was maintained.

The magnetic disk was further subjected to record/reproduction test at700 kFCl, and a sufficient signal-to-noise ratio (S/N ratio) wasobtained with no error.

The magnetic disk was further driven in a 1 inch hard disk driverequiring an information recording density of at least 60 gigabits persquare inch. The magnetic disk performed recording and reproductionsuccessfully without problems. Specifically, no problems resulting fromhead crash or thermal asperities occurred.

Examples 2 to 12, Comparative Examples 1 to 10

A plurality of samples of the magnetic disk glass substrate wereprepared under arbitrarily selected chemical strengthening conditions soas to vary the sectional profile of the stress layers and the waviness(Wa) of the main surfaces of the magnetic disk glass substrate. Thesesamples were used for Examples 2 to 12 and Comparative Examples 1 to 10.The preparation conditions (chemical strengthening conditions) of thesesamples are shown in Table 1, including those of Example 1. The samplesof the magnetic disk substrate of the Examples 2 to 12 and ComparativeExamples 1 to 10 were prepared under the same conditions as in Example 1except for the chemical strengthening conditions.

TABLE 1 Sample preparation conditions (chemical strengtheningconditions) Composition of Chemical Glass Glass Strengthening SaltMixture Example/ Substrate Substrate Potassium Sodium Lithium TreatmentTreatment Comparative Diameter Thickness Content Content ContentTemperature Time Examples (mm) (mm) (vol %) (vol %) (vol %) (° C.)(hour) Example 1 27.4 0.381 59.9995 39.9995 0.001 340 2 Example 2 27.40.381 59.9995 39.9995 0.001 340 4 Example 3 27.4 0.381 59.9 39.9 0.2 3404 Example 4 27.4 0.381 59.9995 39.9995 0.001 380 2 Example 5 27.4 0.38159.9 39.9 0.2 380 2 Example 6 27.4 0.381 59.9995 39.9995 0.001 380 4Example 7 27.4 0.381 59.9 39.9 0.2 380 4 Example 8 27.4 0.381 89.99959.9995 0.001 420 4 Example 9 27.4 0.381 95.7 4 0.3 420 6 Example 10 27.40.381 59.7 39 0.3 420 6 Example 11 27.4 0.381 20.2 79.5 0.3 380 4Example 12 27.4 0.381 49.9995 49.9995 0.001 400 3 Comparative 27.4 0.38159.8 39.8 0.4 340 2 Example 1 Comparative 27.4 0.381 59.8 39.8 0.4 450 4Example 2 Comparative 27.4 0.381 No chemical strengthening treatmentExample 3 Comparative 27.4 0.381 1 99 0 380 4 Example 4 Comparative 27.40.381 90 7 3 420 2 Example 5 Comparative 27.4 0.381 80 20 0 360 4Example 6 Comparative 27.4 0.381 75 24 1 380 4 Example 7 Comparative27.4 0.381 70 30 0 450 4 Example 8 Comparative 27.4 0.381 70 29 1 380 4Example 9 Comparative 27.4 0.381 7 90 3 340 2 Example 10

Pt, Pc, D/2, D/T, L, and L·Pt of the magnetic disk glass substrates inExamples 2 to 12 and Comparative Examples 1 to 10 were obtained fromtheir respective profiles at the sections of the stress layers, in thesame manner as in Example 1. Further, the impact test (3000 G) and themeasurement for waviness (Wa) at the main surfaces were performed on theglass substrates in the same manner as in Example 1. The results areshown in Table 2, including those of Example 1.

TABLE 2 Measured values of Pt, Pc, D/2, D/T, L, and L · Pt of the stresslayer profiles at the sections of the magnetic disk glass substrates,results of impact test on the glass substrates at 3000 G, and measuredvalues of the waviness (Wa) of the main surface of the glass substrateGlass Main Substrate Surface Example/ Impact Waviness Comparative Pt PcD/2 L L * Pt Test at (Wa) Examples (kg/mm²) (kg/mm²) (mm) D/T (mm)(kg/mm) 3000 G (nm) Example 1 3.62 10.10 0.089 0.47 0.203 0.735 OK 0.54Example 2 5.50 11.08 0.100 0.52 0.181 0.996 OK 0.58 Example 3 1.86 5.320.082 0.43 0.217 0.404 OK 0.51 Example 4 5.70 11.89 0.101 0.53 0.1791.020 OK 0.55 Example 5 2.18 4.97 0.091 0.48 0.199 0.434 OK 0.53 Example6 6.22 13.71 0.102 0.54 0.177 1.101 OK 0.54 Example 7 2.44 5.57 0.0970.51 0.187 0.456 OK 0.51 Example 8 6.95 7.58 0.127 0.67 0.277 1.925 OK1.00 Example 9 4.88 5.78 0.150 0.79 0.238 1.161 OK 0.90 Example 10 5.805.58 0.152 0.80 0.272 1.578 OK 0.95 Example 11 2.20 4.31 0.125 0.660.253 0.557 OK 0.51 Example 12 1.30 4.56 0.121 0.64 0.33 0.429 OK 0.55Comparative 1.57 4.65 0.073 0.38 0.235 0.369 NG 0.45 Example 1Comparative 12.55 23.32 0.108 0.57 0.165 2.071 OK 1.05 Example 2Comparative 0 0 0 0 0 0 NG 0.43 Example 3 Comparative 1.01 1.10 0.0730.38 0.201 0.203 NG 0.44 Example 4 Comparative 6.57 3.01 0.078 0.410.342 2.247 OK 1.15 Example 5 Comparative 10.21 9.85 0.154 0.81 0.2963.022 OK 1.32 Example 6 Comparative 12.85 12.54 0.155 0.81 0.187 2.403OK 1.07 Example 7 Comparative 8.59 11.30 0.118 0.62 0.278 2.388 OK 1.08Example 8 Comparative 10.05 25.20 0.167 0.88 0.249 2.502 OK 1.23 Example9 Comparative 0.80 1.10 0.011 0.06 0.2 0.160 NG 0.44 Example 10

The results shown in Table 2 suggest that if {T−(d1+d2)}·Pt, that is,L·Pt, is less than 0.4 (kg/mm), the glass substrate cannot solely endurean impact of 3000 G. It has also been found that the samples having anL·Pt value of more than 2.0 (kg/mm) have a waviness (Wa) of more than1.0 nm at the main surfaces.

FIG. 2 is a plot of the results of Pt and L measurements of eachmagnetic disk glass substrate shown in Table 2. In the figure, blackcircles represent the results on the magnetic disk glass substrates inExamples 1 to 12, black triangles represent the results on the magneticdisk glass substrates in comparative examples that cannot endure animpact of 3000 G, and black squares represent the results on themagnetic disk glass substrates in comparative examples that have awaviness (Wa) of more than 1.0 nm at the main surfaces. The curved solidline represents L·Pt=0.4 (kg/mm), and the curved dotted line representsL·Pt=2.0 (kg/mm).

The magnetic disk prepared in Examples 2 to 12 and Comparative Examples1 to 10 were subjected to glide height test with an AE sensor, and glidetests using a glide head with a flying height of 10 nm in the samemanner as in Example 1. Table 3 shows measured waviness (Wa) at the mainsurfaces of each magnetic disk glass substrate prepared in Examples 2 to12 and Comparative Examples 1 to 10 and the results of the glide heightand glide test in Examples 2 to 12 and Comparative Examples 1 to 10,including the results in Example 1.

TABLE 3 Measured waviness (Wa) at the main surfaces of magnetic diskglass substrates, and results of glide height and glide test on magneticdisks using the substrates Example/ Waviness (Wa) at Main Glide TestComparative Surfaces of Glass Glide Height (Head Flying ExamplesSubstrate (nm) (nm) Height: 10 nm) Example 1 0.54 4.3 OK Example 2 0.584.3 OK Example 3 0.51 4.5 OK Example 4 0.55 5.1 OK Example 5 0.53 4.8 OKExample 6 0.54 6.0 OK Example 7 0.51 4.0 OK Example 8 1.00 9.5 OKExample 9 0.90 6.5 OK Example 10 0.95 6.8 OK Example 11 0.51 5.3 OKExample 12 0.55 4.2 OK Comparative 0.45 4.2 OK Example 1 Comparative1.05 10.2 NG Example 2 Comparative 0.43 3.8 OK Example 3 Comparative0.44 4.3 OK Example 4 Comparative 1.15 11.3 NG Example 5 Comparative1.32 15.3 NG Example 6 Comparative 1.07 10.3 NG Example 7 Comparative1.08 10.3 NG Example 8 Comparative 1.23 11.3 NG Example 9 Comparative0.44 3.9 OK Example 10

FIG. 3 is a plot of the waviness (Wa) at the main surfaces of eachmagnetic disk glass substrate shown in Table 3 and the glide height ofthe magnetic disk using the magnetic disk glass substrate. FIG. 3 showsthat the magnetic disk glass substrate having a waviness (Wa) of 1.0 nmor less at the main surfaces allows the magnetic disk to have a glideheight of less than 10 nm.

Therefore, in order to achieve a magnetic disk glass substrate endurableto an impact of 3000 G and having a waviness (Wa) of 1.0 nm or less atthe main surfaces, and to allow the magnetic disk to have a glide heightof less than 10 nm, the following relationship must be satisfied:

0.4 (kg/mm)≦L·Pt≦2.0 (kg/mm)

If the compressive stress layers of the magnetic disk glass substratehave insufficient thicknesses d1 and d2, the impact resistance isreduced. Accordingly, the following relationship must be satisfied:

L≦0.4 (mm) (where L<T)

From the viewpoint of ensuring compressive stress layers with sufficientthicknesses d1 and d2, the following relationship may hold:

(D/T)≧0.4

Compressive stress layers with excessively large thicknesses d1 and d2may increase the tensile stress excessively in the tensile stress layerto induce delayed fracture. Therefore, the following relationship ispreferably satisfied from a practical viewpoint:

(D/T)≦0.8

Compressive stress layers of the magnetic disk glass substrate having aninsufficient compressive stress Pc reduce the impact resistance.Therefore, the following relationship must be satisfied.

Pc≧4 (kg/mm²)

In order to prevent the increase in waviness (Wa) at the main surfacesof the substrate, and delayed fracture in the substrate, the tensilestress Pt of the tensile stress layer may satisfy the followingrelationship:

Pt≦10 (kg/mm²)

For a margin of impact resistance, magnetic disk glass substrates wereprepared under arbitrarily selected chemical strengthening conditions inthe same manner as above and were subjected to impact resistance test at4000 G. As a result, it was found that magnetic disk glass substrateshaving a L·Pt value of 0.5 or more are endurable to an impact of 4000 G.

Each magnetic disk produced in Examples 2 to 12 was subjected torecord/reproduction test at 700 kFCl in the same manner as in Example 1,and a sufficient signal-to-noise ratio (S/N ratio) was obtained with noerror. Furthermore, the magnetic disk was driven in a 1 inch hard diskdrive requiring an information recording density of at least 60 gigabitsper square inch in the same manner as in Example 1. The magnetic diskperformed recording and reproduction successfully without problems.Specifically, no problems resulting from head crash or thermalasperities occurred.

Moreover, each magnetic disk produced in Comparative Examples 1 to 10was subjected to record/reproduction test at 700 kFCl in the same manneras in Example 1. The magnetic disks produced in Comparative Examples 1,3, 4, and 10 exhibited sufficient signal-to-noise ratio (S/N ratio) withno error, but the magnetic disks produced in the other ComparativeExamples did not exhibit sufficient signal-to-noise ratio (S/N ratio)and produced an error due to unfavorable flying characteristics of thehead.

Furthermore, the magnetic disk was driven in a 1 inch hard disk driverequiring an information recording density of at least 60 gigabits persquare inch in the same manner as in Example 1. The magnetic disksproduced in Comparative Examples 1, 3, 4, and 10 performed recording andreproduction successfully without problems, but the magnetic disksproduced in the other Comparative Example did not appropriately performrecording and reproduction, and caused failures resulting from headcrash or thermal asperities.

The present invention does not limit the diameter (size) of the magneticdisk glass substrate. However, the invention is advantageousparticularly in the manufacture of magnetic disk glass substrates withsmall diameters. The magnetic disk glass substrate with small diametermentioned herein is used for magnetic disks with, for example, adiameter of 30 mm or less.

The small magnetic disk with a diameter of, for example, 30 mm or lessis used for a storage of vehicle-mounted apparatuses such as carnavigation systems or portable apparatuses such as PDA's and mobilephone units, and accordingly requires higher durability and impactresistance than general magnetic disks used for fixed apparatuses.

INDUSTRIAL APPLICABILITY

The magnetic disk glass substrate according to the present invention isused in a hard disk drive for a storage of vehicle-mounted apparatusessuch as car navigation systems or portable apparatuses such as PDA's andmobile phone units.

1. A magnetic disk glass substrate for use in a hard disk drive andhaving a disk thickness of less than 0.5 mm and mirror-finished mainsurfaces, the glass substrate comprising: compressive stress layersformed at the main surfaces; and a tensile stress layer formed betweenthe compressive stress layers, wherein a product of a thickness of thetensile stress layer and a maximum tensile stress of the tensile stresslayer is set at a predetermined value, so that the magnetic disk glasssubstrate has a predetermined impact resistance and the main surface ofthe magnetic disk glass substrate has a predetermined waviness (Wa), andwherein the thickness of the tensile stress layer is measured byobserving a longitudinal section of the magnetic disk glass substratewith a Babinet compensator.
 2. The magnetic disk glass substrateaccording to claim 1, wherein: the product of the thickness of thetensile stress layer and the maximum tensile stress of the tensilestress layer falls within a range of 0.4 to 2.0 kg/mm.
 3. The magneticdisk glass substrate according to claim 1, wherein: the impactresistance is 3000 G or more and, the waviness (Wa) is 1.0 nm or less.4. The magnetic disk glass substrate according to claim 1, wherein: apredetermined disk thickness is set by lapping the main surfaces, andthe mirror-finished surfaces are formed with no cracks by polishing themain surfaces.
 5. The magnetic disk glass substrate according to claim1, wherein: a thickness of the tensile stress layer is 0.4 mm or less,and a maximum tensile stress of the tensile stress layer is 10 kg/mm² orless.
 6. The magnetic disk glass substrate according to claim 1,wherein: a total thickness of the compressive stress layer at one mainsurface and the compressive stress layer at the other main surface is atleast 40% of the disk thickness.
 7. The magnetic disk glass substrateaccording to claim 6, wherein: a maximum tensile stress of the tensilestress layer is 10 kg/mm² or less.
 8. The magnetic disk glass substrateaccording to claim 1, wherein: a highest compressive stress in thecompressive stress layers is 4 kg/mm² or more.
 9. The magnetic diskglass substrate according to claim 1, wherein: the magnetic disk glasssubstrate is used for a magnetic disk installed in a hard disk drivethat starts and stops operation by a load/unload system.
 10. A magneticdisk comprising: the magnetic disk glass substrate according to claim 1,and a magnetic layer formed on the magnetic disk glass substrate.